After months of testing, I concluded that it's impossible to have a higher powerline speed on my house' electrical system. As my HTPC need a fast connection as I use it to play 1080p videos with high bitrate (10mbps-20mbps), I will have to use direct ethernet connection on it. I wont need a faster connection on the HTPC on the same floor as my room since its only used for normal browsing and set-top box use.